fix(app): add friendly not-found handling

packages/backend/convex/agentJobs.ts

@@ -710,7 +710,7 @@ export const get = query({
   handler: async (ctx, { jobId }) => {
     const ownerId = await getRequiredUserId(ctx);
     const job = await ctx.db.get(jobId);
-    if (job?.ownerId !== ownerId) throw new ConvexError('Agent job not found.');
+    if (job?.ownerId !== ownerId) return null;
     return job;
   },
 });